Sound proofing materials and structures have important applications in the acoustic industry. Traditional materials used in the industry, such as absorbers and reflectors, are usually active over a broad range of frequencies without providing frequency selective sound control. Active noise cancellation equipment allows for frequency selective sound attenuation, but it is typically most effective in confined spaces and requires an investment in, and operation of, electronic equipment to provide power and control.
Traditional sound-absorbing materials (for example, foams or fibrous materials) are generally relatively light in weight and porous and serve to dissipate the vibration energy of sound waves over their relatively large surface areas. Helmholtz resonators (comprising, for example, a layer of air sandwiched between two elastic substrates) can also be employed as sound absorbers. For both types of absorbers, however, relatively thick structures are generally required in order to obtain relatively good absorption characteristics at relatively low audible frequencies (for example, approximately 50 millimeters (mm) thickness for frequencies less than about 500 hertz (Hz)), and such thick structures can be problematic for use in confined spaces.
In contrast with the traditional sound-absorbing materials, traditional sound barriers tend to be relatively heavy and air-tight because the sound transmission loss from a material is generally a function of its mass and stiffness. The so-called “mass law” (applicable to many traditional acoustic barrier materials in certain frequency ranges) dictates that as the weight per unit area of a material is doubled, the transmission loss through the material increases by 6 decibels (dB). The weight per unit area can be increased by using denser materials or by increasing the thickness of the barrier. Added weight, however, can be undesirable in many applications.
Phononic crystals (that is, periodic inhomogeneous media, typically in the form of elastic/elastic or elastic/fluid constructions) have been proposed as sound barriers with acoustic passbands and band gaps. Such structures can generate acoustic band gaps in a passive, yet frequency selective way, without having to rely on viscous dissipation or resonance as the leading physical mechanism. Instead, the transmission loss is due to Bragg scattering, which results from the sound speed contrast between the two or more components of an inhomogeneous, multi-phase, spatially periodic structure.
For example, periodic arrays of copper tubes in air, periodic arrays of composite elements having high density centers covered in elastically soft material (to provide an array of localized resonant structures), and periodic arrays of water in air have been proposed to create sound barriers with frequency-selective characteristics. These approaches have typically suffered, however, from drawbacks such as the production of narrow band gaps, the production of band gaps at frequencies too high (for example, ultrasound frequencies of 20 kHz or higher) for audio applications, and/or the need for bulky and/or heavy physical structures (for example, metal pipes having diameters of several centimeters arranged in arrays having external dimensions of decimeters or meters).